EHV-1 Inactivated Vaccine Efficacy Tested (AAEP 2011)

Veterinarians have demonstrated in past studies that the best available vaccines were somewhat effective in limiting nasal shedding, but they were only sometimes effective in preventing development of viremia.

Photo: Anne M. Eberhardt/The Horse

Equine herpesvirus-1 (EHV) is highly prevalent among the horse population due to its ability to persist latently in infected horses for their lifetimes, where it can be reactivated and shed in times of stress. Although veterinarians cannot eliminate this virus entirely from horse populations, they can make efforts toward containment. Immunization is one pivotal strategy in controlling spread of this virus in an equine population, and an immunologist from Colorado State University described efficacy of two available EHV-1 vaccines at the 2011 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas.

In his presentation Paul Lunn, DVM, BVSc, MS, Dipl. ACVIM, professor of equine medicine and head of the Department of Clinical Sciences at CSU's College of Veterinary Medicine and Biomedical Sciences, described the classic characterization of an EHV-1 infection with its two-phase fever spike; an initial fever is associated with nasal shedding and the second fever coincides with viremia (virus circulating in the bloodstream). Then by seven to 12 days, sequelae such as abortion or neurologic signs appear.

Veterinarians have demonstrated in past studies that the best available vaccines were somewhat effective in limiting nasal shedding, but they were only sometimes effective in preventing development of viremia. Lunn commented that vaccines protected best when horses had received boosters recently. Best results also were achieved when vaccines labeled against herpes viral abortion were used--vaccines labeled solely for respiratory herpesvirus prevention have only 30% as much antigen concentration as the anti-abortigenic vaccines.

He described a challenge study he and colleagues performed on three groups of 10 young ponies (2 ½ years old). Each of two groups was immunized with either of two commercially available killed-virus vaccines (Prodigy by Intervet--now Merck Animal Health--and Calvenza by Boehringer Ingelheim). Horses in the third group served as controls and were not vaccinated. The researchers gave the initial two priming doses of herpes vaccine one month apart; they delayed the third vaccine in the primary series another three months, and then they subjected the ponies to viral challenge one month after the third immunization in the series.

The challenge experiment did not produce extensive disease, which Lunn suggested could in part be due to the ponies' young age. While clinical scores, rectal temperatures, viral shedding, and viremia for the three groups were similar, investigators noted statistical differences in virus neutralization titers between the vaccination groups: Calvenza-immunized ponies had higher titers than the Prodigy-immunized group, which were higher than the nonvaccinated control ponies.

Lunn concluded, "There was no significant reduction of clinical signs or nasal shedding by either vaccine, only a trend." He emphasized that the best killed and live vaccines are somewhat effective only if regular vaccine regimes are implemented along with timely boosters.

While currently available vaccines might not protect directly against neurologic sequelae to herpesvirus infection, such as equine herpes myeloencephalopathy, there is value in using the highly antigenic herpes vaccines. Their value lies in reducing nasal shedding of virus and, hence, exposure within a herd or population of horses that are maintained on a regular herpes immunization schedule.

About the Author

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado,
and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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